Effect of Molybdenum Content on the Hardenability and Precipitation Behaviors of Boron Steel Austenitized at High Temperatures

The effect of molybdenum (Mo) contents on hardenability and precipitation behaviors in Mo-B simultane-ously added steels were investigated placing a focus on high austenitizing temperature. The hardenability of 0.5% Mo - 11 ppm B steel austenitized at 1 150 degrees C was decreased compared with that austenitized at 950 degrees C, whereas 1.0% Mo - 10 ppm B and 1.5% Mo - 9 ppm B steels were less affected by high austenitizing tem-perature than 0.5% Mo - 11 ppm B steel. The Fe23(C, B)6 precipitation by increasing austenitizing temperature was also revealed to be suppressed in 1.5% Mo - 9 ppm B steel. These results indicate that the improved effect of the Mo addition on hardenability by retarding the precipitation of Fe23(C, B)6 still appear in B-added steels austenitized at high temperature. Furthermore, Fe23(C, B)6 precipitation start temperature was increased in Mo-B added steels austenitized at 1 150 degrees C. This result implies that non-equilibrium B segrega-tion mechanism during cooling from high austenitizing temperature enhances the amount of segregated B on grain boundaries leading to the promotion of the borides precipitation at high austenitizing temperature region. However, Mo is presumed to fix a part of thermal vacancies as Mo-V complex resulting in the sup-pression of non-equilibrium B segregation to grain boundaries during cooling, which is speculated to inhibit the Fe23(C, B)6 precipitation. Thus, the effect of Mo-B combined addition on hardenability was presumably maintained even in high austenitizing temperature region